1. Field of the Invention
The invention relates to a wind rotor for a windmill, which wind rotor has airfoil blades or wings, each of which is provided with an aerodynamic brake which may be activated upon exceeding a predetermined rotational speed of the wind rotor, which brake consists of a turnable part of the wing and which turnable part preferably is the radially outermost part of the wing.
2. Related Art
A windmill extracts energy from the wind force acting on the blades of the wind rotor and transforms the energy of the wind to electrical, mechanical, or other useful form of energy. The type of windmill for which the invention most advantageously can be put to use is a windmill with a horizontal or approximately horizontal wind rotor axis and having an arbitrary number of wings rotating in an approximately vertical plane perpendicular to the direction of the wind.
To qualify for an operating permit, a windmill must be provided with two braking systems, each on its own being able to prevent the rotor of the windmill from exceeding a specified rotary speed. One of the two braking systems is usually a mechanical brake; the other may conveniently be an aerodynamic braking system which is activated in response to an excessive speed and which through the braking effect of the forces of the air reduces the rotor speed to a safe value. The excessive speed may occur, for instance, if the mechanical brake of the windmill fails when actuated to stop the windmill. The aerodynamic brake may also, besides this primary emergency function, act together with the mechanical brake during the normal operation of the windmill. However this demands that the brake be operated by the control system of the windmill.
Various types of aerodynamic controls and brakes on wind rotors for windmills are known, and the following may be mentioned here:
a) Pitch control
The whole wing may be turned around its longitudinal axis and used to control the windmill. Pitch control is a different function than aerodynamic braking, which is the concern of the present invention.
b) Brakinq Parachute
A braking parachute is placed in a collapsed state in a space in the wing tip, i.e., the radially outermost part of the wing. The use of a braking parachute implies two problems; first that it is time consuming to fold up and again deposit the parachute in the space after it has been in use, and second that it is problematic strengthwise to use braking parachutes in connection with long wings because the braking will be very sudden.
c) Air brakes
Air brakes consist of plates which are inset into the wing and which may be turned into or shifted into the air stream. This type of brake is similar to the brakes used in gliders. Experience with such brakes on windmills shows that they are not sufficiently effective unless they are very long in proportion to the length of the wing and therefore disproportionately expensive.
d) Leading edge flap
On the radially outer approximately 15% of the length of the wing, a leading part, which may include 15-20% of the chord of the wing, is hinged longitudinally to the main part of the wing such that it may be pivoted to a position almost perpendicular to the wing surface.
It is a problem to construct the hinge parts of a leading edge flap so that it will accurately return to the correct stowed position in relation to the main part of the wing behind it, as only a small departure from the designed airfoil profile may produce considerable changes in the aerodynamic characteristics of the wing.
e) Swing tip
The radially outer part of the wing, the tip, which typically constitutes the outer 15% of the total length of the wing, may be rotatable on an axle which extends in the longitudinal direction from the main part of the wing approximately at the middle of the wing chord. When the swing tip is activated, it is turned to a position approximately perpendicular to the rotational plane of the wing. The mounting of this axle presents a number of problems, and at the interface between the main part of the wing and the swing tip the axle is subject to high bending moments. This makes a large diameter of the axle necessary, which involves a corresponding increase in the thickness of the wing.